Abstract

In this study, dimension control and nanostructure design were applied in developing electrode materials for superior Na-ion storage. Firstly, hierarchical Co₃O₄nanorods with increased number of interfaces between individual nanocrystallites were prepared for enhancing pseudocapacitance assisted Na-ions storage. When evaluated as anode materials for Sodium ion batteries, Co₃O₄nanorods exhibited excellentelectrochemical performance with specific capacities of 644 mAh/g at 25 mA/g and 225 mAh/g at 1 A/g. These values are significantly better than those of high surface area Co₃O₄nanoparticles. This can be ascribed to the synergistic effect of conversion reaction and interfaces induced pseudocapacitance for the superior Na-ions storage. Moreover, crevices generated between adjacent nanoparticles can facilitate faster Na-ion diffusion, and accommodate volume changes during conversion reaction, leading to an enhanced rate stability compared to nanoparticles electrode. Post-cycling microscopic analysis demonstrated excellent structural stability of the hierarchical nanorods after extended charge-discharge cycles.

Secondly, CoO nanoparticles anchored on three-dimensional reduced graphene oxides (RGO) (CoO@3D-RGO) were prepared through facile hydrothermal method and subsequently heat-treatment. Ultrafine CoO nanoparticles are well deposited on the surface of 3D-RGO networks, which effectively reduce the stacking of RGO sheets and maintain a high active surface area. When tested as Na-ion battery anode material, CoO@3D-RGO electrode delivered a high specific capacity of 401 mAh/g at 25 mA/g, and 149 mAh/g at 1 A/g. The improved electrochemical performance is ascribed to the 3D morphology, large accessible area, rapid ion transport ability, and low charge transfer resistance of the CoO@3D-RGO. Na-ion storage mechanism consists of both conversion and pseudocapacitance mechanism.Post-cycling microscopic studies confirmed the retention of 3D morphology during long-term charge-discharge cycles.